For parallel building to be successful, the dependencies among files must
be properly spelled out, as targets tend to get built in a quickest-first
ordering. Also, beware of un-parallelizable commands that drop fixed-named
files into the current directory, like yacc(1) does.

This section is derived from the official Jam documentation and from experience
using it and reading the Jambase rules. We repeat the information here mostly
because it is essential to understanding and using Jam, but is not consolidated
in a single place. Some of it is missing from the official documentation
altogether. We hope it will be useful to anyone wishing to become familiar
with Jam and the Boost build system.

Jam "rules" are actually simple procedural
entities. Think of them as functions. Arguments are separated by colons.

A Jam target is an abstract entity identified
by an arbitrary string. The build-in DEPENDS rule
creates a link in the dependency graph between the named targets.

Note that the original Jam documentation for the built-in INCLUDES
rule is incorrect: INCLUDES targets1 :
targets2 causes everything that depends
on a member of targets1 to depend on all members
of targets2. It does this in an odd way, by tacking
targets2 onto a special tail section in the dependency
list of everything in targets1. It seems to be OK
to create circular dependencies this way; in fact, it appears to be the
"right thing to do" when a single build action produces both
targets1 and targets2.

When a rule is invoked, if there are actions declared
with the same name as the rule, the actions are added to the updating
actions for the target identified by the rule's first argument. It is
actually possible to invoke an undeclared rule if corresponding actions
are declared: the rule is treated as empty.

Targets (other than NOTFILE targets) are associated
with paths in the file system through a process called binding. Binding
is a process of searching for a file with the same name as the target
(sans grist), based on the settings of the target-specific SEARCH
and LOCATE variables.

In addition to local and global variables, jam allows you to set a variable
on a target. Target-specific variable values can usually
not be read, and take effect only in the following contexts:

In updating actions, variable values are first looked up on
the target named by the first argument (the target being updated).
Because Jam builds its entire dependency tree before executing
actions, Jam rules make target-specific variable settings as a
way of supplying parameters to the corresponding actions.

Binding is controlled entirely by the target-specific
setting of the SEARCH and LOCATE
variables, as described here.

In the special rule used for header file scanning, variable values
are first looked up on the target named by the
rule's first argument (the source file being scanned).

The "bound value" of a variable is the path associated with
the target named by the variable. In build actions, the first two arguments
are automatically replaced with their bound values. Target-specific variables
can be selectively replaced by their bound values using the bind
action modifier.

Note that the term "binding" as used in the Jam documentation
indicates a phase of processing that includes three sub-phases: binding
(yes!), update determination, and header file scanning. The repetition
of the term "binding" can lead to some confusion. In particular,
the Modifying Binding section in the Jam documentation should probably
be titled "Modifying Update Determination".

"Grist" is just a string prefix of the form <characters>.
It is used in Jam to create unique target names based on simpler names.
For example, the file name "test.exe" may
be used by targets in separate subprojects, or for the debug and release
variants of the "same" abstract target. Each distinct target
bound to a file called "test.exe" has its own unique grist
prefix. The Boost build system also takes full advantage of Jam's ability
to divide strings on grist boundaries, sometimes concatenating multiple
gristed elements at the beginning of a string. Grist is used instead
of identifying targets with absolute paths for two reasons:

The location of targets cannot always be derived solely from what
the user puts in a Jamfile, but sometimes depends also on the binding
process. Some mechanism to distinctly identify targets with the
same name is still needed.

Grist allows us to use a uniform abstract identifier for each built
target, regardless of target file location (as allowed by setting
ALL_LOCATE_TARGET).

When grist is extracted from a name with $(var:G), the result includes
the leading and trailing angle brackets. When grist is added to a name
with $(var:G=expr), existing grist is first stripped. Then, if expr is
non-empty, leading <s and trailing >s are added if necessary to
form an expression of the form <expr2>; <expr2> is then prepended.

When Jam is invoked it imports all environment variable settings into
corresponding Jam variables, followed by all command-line (-s...) variable
settings. Variables whose name ends in PATH, Path, or path are split
into string lists on OS-specific path-list separator boundaries (e.g.
":" for UNIX and ";" for Windows). All other variables
are split on space (" ") boundaries. Boost Jam modifies that
behavior by allowing variables to be quoted.

A variable whose value is an empty list or which consists entirely of
empty strings has a negative logical value. Thus, for example, code like
the following allows a sensible non-empty default which can easily be
overridden by the user:

MESSAGE?\=startingjam...;if$(MESSAGE){ECHOThemessageis:$(MESSAGE);}

If the user wants a specific message, he invokes jam with "-sMESSAGE=message
text". If he wants no message, he invokes jam with -sMESSAGE=
and nothing at all is printed.

The parsing of command line options in Jam can be rather unintuitive,
with regards to how other Unix programs accept options. There are two
variants accepted as valid for an option: